In communication technology, the communication data are conventionally modulated onto a signal and transmitted over a transmission channel. At a receiver end, the communication data are then recovered from the received signal. The amplitude of the received signal has an amplitude attenuation that is dependent on the length of the transmission path. Because the length of the transmission path, and thus the amplitude attenuation, are generally unknown, the amplitude of the received signal is consequently uncertain. In order to demodulate the communication data from the received signal, a variable gain amplifier (VGA) circuit is therefore included in the receiver to amplify the received signal. The VGA circuit conventionally operates in an automatic gain control (AGC) loop so as to generate a signal with a constant amplitude value independently of the received signal.
FIG. 1 shows a typical circuit diagram of a prior art receiver 100. The receiver 100 mainly includes an antenna 101, a low-noise amplifier (LNA) 103, a mixer 105, a local oscillator circuit (OSC) 107, a band pass filter (BPF) 108, a VGA circuit 109, an analog to digital converter (ADC) 111, and a demodulator 113. A high frequency signal that is modulated by the communication data is firstly received by the antenna 101, and then amplified by the LNA 103, mixed with a reference wave from the OSC 107 by a mixer 105, and filtered by the BPF 108. An intermediate frequency (IF) signal IF_IN is provided from the BPF 108 to the VGA circuit 109. The VGA circuit 109 further amplifies the IF signal IF and provides an amplified signal IF_OUT. Then, the ADC 111 converts the amplified signal IF_OUT from the VGA circuit 109 to a digital signal and the demodulator 113 demodulates the communication data from the digital signal.
FIG. 2 shows a circuit diagram of the VGA circuit 109 in FIG. 1. The VGA circuit 109 includes an amplifier unit 102 that consists of a variable gain amplifier 110 and a fixed gain amplifier 112, and an AGC loop 104 that consists of a peak detector 114, a comparator 116 and a loop filter 118. In the amplifier unit 102, the IF signal IF_IN from the BPF 108 firstly passes through the variable gain amplifier 110 and then through the fixed gain amplifier 112. Through the amplifier unit 102, the IF signal IF_IN is translated into the output signal IF_OUT with a predetermined gain. Furthermore, the level of the output signal IF_OUT is further stabilized by the AGC loop 104. Thus, the output signal IF_OUT obtains a constant and optimal voltage level that satisfies the dynamic range requirement of the ADC 111.
As shown in FIG. 2, one configuration of the AGC loop 104 is that the peak detector 114 receives the output signal IF_OUT from the output terminal of the fixed gain amplifier 112 and then provides a peak signal that indicates the peak of the output signal IF_OUT. The peak signal is then compared with a reference signal Vref by the comparator 116, which generates a control signal according to the comparison. After being filtered by the loop filter 118, the control signal is then used to regulate the gain of the variable gain amplifier 110. The AGC loop 104 will continue to regulate the gain until the peak signal is identical to the reference signal Vref. When the gain regulation completes, the output signal IF_OUT becomes constant.
Considering the input voltage range requirement of the peak detector 114, another configuration (not shown) of the AGC loop is conceived. In this configuration, the peak detector 114 is connected to the input terminal of the fixed gain amplifier 112. The remaining circuitry of the AGC circuit is the same as shown in FIG. 2.
However, performance of the peak detector 114 tends to change with process, voltage and temperature (PVT) variation. Thus, for both configurations of the AGC loop described above, the output signal IF_OUT cannot be accurately controlled. Furthermore, conventional fixed gain amplifier is also vulnerable to the PVT variation and therefore undermines the constancy of the output signal IF_OUT.
Therefore, it is desirable to have a circuit and method that does not suffer from the PVT variation and maintains the output signal constant and optimal. It is to such a circuit and method the present invention is primary directed.